Portable devices often need to be synchronized with a reference system. For example, portable Global Positioning Systems (GPS), derive position data from signals transmitted from satellites. The portable GPS device uses a clock to obtain a position fix with a GPS satellite. When the GPS signals are not available, for example, because the satellites are out of view, or when the mobile device has not acquired the satellite signal, the mobile device must be resynchronized with a reference clock due to drift in the mobile unit clock over time.
Network delay is defined as the time required to send synchronization information over a network between a reference station and a remote station. In order to accurately resynchronize a remote clock with a reference clock, it is necessary to account for this network delay.
This patent application describes a system for resynchronizing a clock in a portable device by sending a pulse from a reference station to the portable device. The portable device relays a pulse back to the reference station. The reference station measures the time difference between the transmitted pulse and the received pulse. This time difference is then used to resynchronize the clock in the portable device.
The present invention provides another technique for identifying network delay.